Multiplex telephony system



NOV. T18, 194l. T s, SKlLLMAN K 2,263,369

MULTIPLEX TELEPHONY SYSTEM Filed Jan. 8, 1940 fd .S/ NHA CIRCUIT /VrTo/P/VEY Patented Nov. 18, 1941 UNITED STATES PATENT OFFICE lands, assignor, by mesne assignments, to Hartford National Bank and Trust Company, Hartlord, Conn., as trustee Application January 8, 1940, Serial No. 312,985 In Germany February 3, 1939 3 Claims.

It is well known in carrying out simultaneous transmission of a number of telephone conversations via the same medium, for example a common transmission line, to use a mechanical or electrical switch which successively and periodically connects each of the channels to the transmission line or equivalent.

In such a system each of the conversations is transmitted in the form of periodical short pulses whose amplitude is modulated in accordance with the conversation to be transmitted.

The invention has for its object to transmit signalling currents, such as calling currents and selecting pulses, in a simple manner in systems of this kind.

According to the invention, for this purpose the periodical pulses which are modulated in amplitude during and by reason of the conversation to be transmitted are wholly or partly suppressed in a predetermined rhythm corresponding to the signals to be transmitted.

The invention is based upon recognition of the fact that even at the time when a conversation is not transmitted in a given channel the periodical pulses are nevertheless emitted in this channel and this permits of emitting a signal in the transmitter by suppression of the said pulses.

This is the case, for example, ln systems employing a cathode-ray tube as the switch. The cathode-ray beam which acts as a switching element periodically establishes connection between a channel and the common transmission line even when there is no conversation in this channel and consequently brings about periodical currentpulses in the transmission line. Signalling currents can be transmitted by suppression of the said pulses in a predetermined rhythm.

If the switch used is constituted by a mechanical switch which successively and periodically connects the various speech channels to the transmission line pulses are generally not emitted during the time when there is no conversation in a given channel. If, however, a source of constant E. M. F. is included in each of the speech channels pulses are emitted in this system even if there is no speech transmission.

In order that the invention may be clearly understood and readily carried into eect l shall describe the same in more detail with reference to the accompanying drawing in which:

Figure l is a schematic diagram of one of the two nal apparatus of a multiplex telephone system employing a cathode-ray tube as the commutator, and

Fig. 2 is a schematicdiagram of a portion of a multiplex system according to another embodiment of the invention.

The system shown in Figure 1 comprises two cathode-ray tubes 1 and 23, but the connections to these tubes are shown for only one channel, those for the other channels being arranged in similar manner. A subscriber line I is connected to separated transmitting and receiving circuits 20 and 2| respectively by means of a fork circuit comprising a differential transformer having four windings 2 and two windings 26, and a line balance 3.

The cathode-ray commutator tube 1 has a grounded cathode I4, a pair of deflecting plates I'I, a plurality of control electrodes 6, a control electrode screen IB provided with a plurality of apertures I5 each aligned with one of the electrodes 6, and an anode I8.

Transformer 2 has a tap 4I connected through a conductor 5 to the first of the control electrodes 6. and a tap 8 connected to ground through a part of a resistance 9, a conductor Ill, a normally closed contact II of a relay I2, and a portion 50 of a resistance I3 which is connected in parallel with a source of bias voltage 60. The cathode I4 of tube I is grounded and therefore, the voltage drop across portion 50 of resistance I3 controls the bias of the rst of the control electrodes 6 of this tube. When the subscriber connected to the end of the subscriber line I is talking, the speech voltages occurring between points 4I and 8 are superimposed on the abovementioned bias voltage so that the cathode-ray beam, which is periodically reciprocated by a voltage applied to the deecting plates Il, is modulated in accordance with the instantaneous valu-e of the voltage at each of the control electrodes 6 each time the beam passes through one of the apertures I5.

As a result there occurs in the output circuit connected to the anode I8, periodical pulses whose amplitude is modulated in accordance with the conversation received over conductors i. Similarly, pulses are successively set up in the other channels (not shown) which are connected respectively to the other control electrodes 6 of the cathode-ray tube l. After being amplified by an amplier I9, these pulses are emitted via a transmission line 20 to whose other end the receiving circuit of the other final apparatus is connected.

The ilnal apparatus is constituted similarly to that shown in Fig. 1 so that the description of the signal circuit of the nal apparatus shown in Fig. 1 suffices for the description of the signal circuit of the final apparatus at the other end of the transmission line 20, it being assumed that pulses of the form just described, which are emitted in the channel under consideration by the transmitting part of the nal apparatus at the other end of the transmission line 20, are received via a receiving line 2|,

These pulses are supplied to a control electrode 22 of a second cathode-ray tube 23 in a manner similar to the pulses -emitted in the other channels, and thus modulate the intensity of the cathode-ray beam generated in tube 23. The tube 23 also comprises a grounded cathode 5I, a pair of deliecting plates 29 and a plurality of anodes 24, only the first of which is connected in the figure. The beam of tube 23 is periodically reciprocated by a voltage applied to the deflecting plates 29 in such a manner that each time a pulse is received in the channel under consideration the cathode-ray beam impinges on the iirst of the anodes 24, i. e. the uppermost anode in the gure.

The iirst anode 24 is connected to ground through a transmission line 25, the windings 26 of the differential transformer, 'and condensers 21 and 28. As the cathode 5I is also grounded, a current modulated in accordance with the conversation to be transmitted passes through windings 26 when the cathode-ray beam impinges on the rst anode 24 with the result that a voltage is induced in windings 2 and the conversation is fed to the subscriber via the subscriber line I. 'Ihe movements of the cathode-ray beams in tube 23, and in the cathode-ray tube (not shown) in the transmitting circuit of the iinal apparatus at the other end of the transmission lines 20 and 2|, are in synchronsm and have a phase displacement corresponding to the transit time of the pulses from one final apparatus to the other. This movement may be brought about, for example by applying to the deiiecting plates I1 and 29 respectively a sawtooth voltage produced by a generator 30.

Even when there is no conversation entering via the subscriber line I and no modulating voltage is applied to the iirst control electrode 6 of cathode-ray tube 1, a pulse is nevertheless emitted in the output circuit connected to the common anode I8 each time the cathode-ray beam passes through the first aperture I of screen I 5 and strikes the anode I8. This also applies to the transmission in the reverse direction so that pulses are also received via the conductor 2l when there is no conversation in this direction in the channel considered. Consequently, a current composed of successive pulses passes in the circuit which is connected to the iirst anode 24. The direct current component of these pulses excites a relay 33 to open its armature 34.

According to the invention and for the purpose of transmitting signal currents, the pulses emitted in the absence of a conversation are suppressed in a given rhythm. This suppression of the pulses is effected by relay I2 which is excited by signal currents passing through a conductor 3l and has an armature 52. The armature 52 is adapted to engage the normally open contact 32 and thus the negative bias of the first control electrode 6 is varied in such manner that the cathode-ray beam is suppressed at the moments during which this beam falls through the first aperture I5. If signalling is brought about similarly in the opposite direction in the channel considered, an impulse is not received at the control electrode 22 during the periods during which the cathode-ray beam strikes the rst anode 24 of cathode-ray tube 23, and in this case the voltage of the electrode 22 is governed solely by its bias which is so selected that the cathode-ray beam is suppressed. Therefore, when the pulses are suppressed for the signal transmission,4 no current passes I through the circuit connected to the first anode 24 so that the relay 33 is periodically deenergized with the result that its contact 34 is opened and closed and signals are transmitted over conductor 35.

Figure 2 shows a system in which mechanical switches 44 and 45 are used as commutators. Switches 44 and 45, which are driven in synchronism and are interconnected by a transmission line 45, comprise as many contacts as there are conversations to'be transmitted, i. e. from 36 to 40 and so forth to 39 to 43, and they successively and periodically connect each of the conversations to the transmission line 46 for a short moment. As shown each switch has four contacts.

When there is no conversation in any of the channels, for example in channel 36-40, impulses are not emitted during the time periods in which any one of the channels is connected to the transmission line 46, so that in this system signalling according to the invention cannot be employed unless additional means are employed. In accordance with the invention I make this possible, by providing means to produce an E. M. F. in the speech channels. More particularly, and for the channel 36-40, a battery 41 is inserted between the conductors of the channel 36-40 with the interposition of a resistance 48 and a normally-closed switch 49. As a result a voltage is applied to these conductors so that each time switch 44 connects this channel to the transmission line 46, a voltage impulse is transmitted. When speech is carried on in the channel 36--40, the speech voltages are superimposed on the constant voltage of the battery 41 and the pulses transmitted via the transmission line 46 are modulated in accordance with the conversation to be transmitted as regards its amplitude.

For signalling, switch 49 is opened and closed in a prescribed rhythm so that pulses are not fed through the transmission line into the channel 36-40 with the result that the mean direct current, lwhich otherwise passes through this channel, is suppressed. As shown the receiver 40 includes a relay 50 which is normally energized by the mean direct current which is interrupted during signalling. The armature of the relay thus engages a contact 5I so that the signal circuit 52 is closed.

Although I have described my invention with reference to certain applications and by means of speciiic examples I do not desire to be limited thereto as obvious modifications will appear to one skilled in the art.

What I claim is:

l. In a multiplex telephone system having a plurality of channels and a common transmission line at the transmitter and receiver sides, switching means for successively and periodically connecting each speech channel to the transmission line, means associated with said switching means for producing in each channel when connected by the switching means impulses which are modulated as regards their amplitudes and by reason of the conversation to be transmitted, and signalling means for at least partially suppressing the impulses in a given rhythm, said latter means including a circuit containing a signalling switch and a source of voltage.

2. In a multiplex telephone system having several channels and a transmission line common thereto, commutating means on the transmitter side including a cathode-ray tube. said tube comprising a common anode connected to said transmission line and aplurallty of control electrodes each connected to one o! the speech channels, commutating means on the receiver side including a cathode-ray tube comprising a control electrode connected to the transmission line and a plurality of anodes each connected to one of the speech channels, means on the transmitter side for successively modulating the beam of the first tube by voltages applied to the control electrodes thereof, means on the receiver side for deiiecting the beam of the rst tube and to cause the same to successively impinge upon the anode's thereof, and means to transmit signals in a given speech channel by varying the bias of the control electrode connected to this channel of the second tube, said latter means including a circuit connected to the corresponding anode of the flrst tube and including a relay adapted to be energized by the mean direct current passing through this circuit and controlling the signal circuit.

3. In a multiplex telephone system having a plurality of channels and a common transmission line at the transmitter and receiver sides, mechanical switching means for successively and periodically connecting each speech channel to the transmission line, a source of voltage in each of the speech channels on the receiver side, means to cut said sources out of circuit during the transmission of signals, a signal circuit, a relay on the receiver side and adapted to close said signal circuit when the voltage source is cut out of circuit.

THOMAS SAMUEL SKILLMAN. 

